Within the framework of the present invention, a flight trajectory comprises:                a lateral trajectory comprising a plurality of successive lateral branches, which are rectilinear and each present a particular course; and        a vertical trajectory comprising a plurality of successive vertical segments, which are rectilinear and each present a particular slope.        
When the aircraft, for example a military transport plane, is guided along such a flight trajectory, the pilot is not forewarned of a change of trajectory, namely either a lateral change (corresponding to a change of lateral branch), or a vertical change (corresponding to a change of vertical segment). Hence, in particular when the pilot pilots the aircraft on the basis of orders provided by a flight director, he is not in a position to anticipate such a change of trajectory and must immediately follow the new orders provided by the flight director upon each change of trajectory. Such flight control is therefore very demanding for the pilot, especially at low altitude where the branch/segment distribution is random and very frequent.